diff --git a/Makefile b/Makefile
index 4562d78..cb4f7d7 100644
--- a/Makefile
+++ b/Makefile
@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS    ?= -W
+SPHINXOPTS    ?= -W -a
 SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = source
 BUILDDIR      = build